For the most part, HTML document tags are simple to understand and use since
they are made up of common words, abbreviations, and notations. Every HTML tag
consists of a tag name, sometimes followed by an optional list of tag
attributes, all placed between opening and closing brackets (< and >).
The simplest tags are nothing more than the tag name enclosed in brackets, such
as <head> and <i>. More complicated tags have attributes, which
may have specific values defined by the author to modify the behavior of a tag.

Tag attributes belong after the tag name, each separated by one or more tab,
space, or return charactesr. The order of attributes in a single tag is not
important. An attribute's value, if it has one, follows an equal sign after
the attribute name. If an attribute's value
is a single word or number, you may simply add it after the equal sign.
All other values should be enclosed in single or double quotation marks,
especially if they contain several words separated by spaces. The length
of an attribute's value is limited to 1024 characters.
Here are some examples of tags with attributes:

Tag and attribute names are not case-sensitive, but attribute values can
be.
For example, it is especially important to use the proper
capitalization when referencing the URLs of other documents with
the href attribute.

Most HTML tags consist of start and end tags that enclose text and other
elements of a document. An end tag is the same as a start tag except
it has
a forward slash (/) before the tag name. End tags never contain
attributes. For example, to italicize text,
you enclose it within the <i> tags:

<i>This text in italics.</i>

You should take care when nesting tagged elements in a document. You must
end nested tags starting with the most recent one and work your way back
out. In this example, a phrase in bold (<b>) appears in the text
of a link (<a href=...>) contained in some body text:

<body>
This is some text in the body, with a
<a href="another_doc.html">link, a portion of which
is <b>set in bold</b></a>
</body>

There are a handful of HTML tags that do not have end tags because they
are standalone elements. For example, the image tag (<img>) inserts a single
graphic into a document and does not require an end tag. Other standalone tags
include the linebreak (<br>), horizontal rule (<hr>), and
tags that provide information about a document that doesn't affect its displayed
content such as the <meta> and <base> tags.

In some cases, end tags can be omitted in a document. Browsers often assume
the end of one element when another begins. The most common example
of this is with the paragraph tag (<p>). Since it is so often used
in a document, a <p> tag usually only appears at the beginning of
each paragraph. When one paragraph ends, another <p> tag signals
the browser to end the paragraph and start another. Most authors do not
use an end paragraph tag. There are other end tags that browsers function
fine without, such as an ending </html> tag. However, it is best
to include the end tags as much as possible to avoid confusion and mistakes
in displaying your document.